This invention relates to high intensity discharge lamps utilizing a time fuse device, and more particularly, to a high pressure sodium vapor lamp having a time fuse device arranged within the lamp so that the life of the HPS lamp is predeterminedly established.
High pressure sodium vapor lamps have been widely used during the past decade for commercial lighting applications, especially outdoor lighting. Such lamps are described in U.S. Pat. No. 3,248,590 of Schmidt, entitled "High Pressure Sodium Vapor Lamps" and which is assigned to the same assignee as the present invention. High pressure sodium lamps typically utilize a slender, tubular inner envelope formed of a light-transmissive refractory oxide material which is resistant to sodium at high temperatures and which is advantageously a high density polycrystalline alumina ceramic. The ceramic arc tube is generally supported within an outer vitreous envelope or jacket provided at one end with the usual screw base. The electrodes in the inner envelope are connected to terminals of the base, that is to shell and center contacts. The space between the inner and outer envelopes is typically evacuated in order to conserve heat.
High pressure sodium lamps typically manifest a normal long term failure termed "lamp cycling" due to lamp voltage rise with time. This lamp cycling can cause the lamp to operate intermittently, resulting in a failure mode which is highly annoying to anyone trying to perform a task under the lamp. Inasmuch as lamp cycling may occur only once every several minutes, lamps in need of replacement manifesting a lamp cycling failure on highways are difficult to spot without a great deal of monitoring time.
In addition to lamp cycling failure, high pressure sodium (HPS) lamps may burn well beyond their rated life at efficiencies less than 70% of their initial value. The operation of HPS lamps below their rated efficiency is disadvantageous in that the lumen output thereof is less than desired and over a prolonged period of time such operation is not cost effective.
It is desired that the long term failure mode of lamp cycling be reasonably predicted and the efficient operation of the HPS lamps be predeterminately established so that appropriate replacement procedures may be implemented to eliminate the bothersome and inefficient operation of HPS lamps.
Accordingly it is an object of the present invention to provide means to eliminate the operation of cycling HPS lamps operating in a cycling failure mode and at efficiencies less than desired.
This and other objects of the present invention will become more apparent upon consideration of the following description of the invention.